Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
  • H04L27/2668—Details of algorithms
  • H04L27/2673—Details of algorithms characterised by synchronisation parameters
  • H04L27/2675—Pilot or known symbols
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
  • H04B1/7073—Synchronisation aspects
  • H04B1/7075—Synchronisation aspects with code phase acquisition
  • H04B1/7077—Multi-step acquisition, e.g. multi-dwell, coarse-fine or validation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/0007—Code type
  • H04J13/0022—PN, e.g. Kronecker
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/10—Code generation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2602—Signal structure
  • H04L27/261—Details of reference signals
  • H04L27/2613—Structure of the reference signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2602—Signal structure
  • H04L27/261—Details of reference signals
  • H04L27/2613—Structure of the reference signals
  • H04L27/26134—Pilot insertion in the transmitter chain, e.g. pilot overlapping with data, insertion in time or frequency domain
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
  • H04L27/2657—Carrier synchronisation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
  • H04L27/2662—Symbol synchronisation

Definitions

• the present invention relates to a method for generating a preamble sequence using a pseudo noise (PN) sequence, and a method for obtaining time synchronization and estimating frequency offset based on the PN sequence in an Orthogonal Frequency Division Multiplexing (OFDM) communication system. More particularly, the present invention concerns a method for generating a preamble sequence based on a PN sequence, and a method for achieving time synchronization and estimating frequency offset in an OFDM communication system.
• PN pseudo noise
• OFDM Orthogonal Frequency Division Multiplexing
• a transmitter part of the OFDM communication system generates a preamble signal in frequency domain so as to make the PN sequence have good autocorrelation properties in a time domain, and a receiver part of the OFDM communication system obtains a symbol synchronization and estimates a subcarrier frequency offset in the time domain by using the PN sequence to generate the preamble sequence.
• Orthogonal Frequency Division Multiplexing is a digital transmission scheme that is efficient in use of bandwidth.
• the OFDM is widely used in digital transmission systems, such as digital audio broadcasting or digital video broadcasting in Europe, Asymmetric Digital Subscriber Line (ADSL), Wireless Local Area Network (WLAN), and so on.
• the OFDM is robust against inter symbol interference (ISI) that is an important issue in the high-speed communication.
• ISI inter symbol interference
• the OFDM can make a frequency selective fading appear as a frequency non-selective fading.
• OFDM communication system is sensitive to the mismatch of oscillators in the receiver and the transmitter, or carrier frequency error caused by Doppler frequency shift.
• the carrier frequency error destroys the orthogonality between the subcarriers in the OFDM communication system, causing intercarrier interference (ICI). Therefore, the frequency synchronization problem that may cause great performance degradation even at a small carrier frequency error is the most important issue in the implementation of the OFDM communication system.
• ICI intercarrier interference
• the symbol synchronization in the OFDM communication system is different from that in the single frequency system.
• the time synchronization in the OFDM communication system means finding a starting estimation point of an OFDM symbol.
• the OFDM communication system generally uses a cyclic prefix (CP), it is less sensitive to the symbol synchronization error, but the symbol synchronization has to be estimated within the CP without departing the error.
• CP cyclic prefix
• OFDM synchronization methods may be classified into a blind method and a data dependent method.
• the data dependent method uses two pilot symbols for the time synchronization and the estimation of both integer and fraction parts of the frequency offset.
• this method has a problem in that data rate decreases because two pilot symbols are used.
• the blind method uses the CP for the time synchronization and the estimation of the fraction part of the frequency offset.
• This method uses the correlation of a data symbol part and a CP part.
• this method cannot estimate the integer part of the frequency offset.
• this method can achieve the time synchronization, it cannot find the beginning of a frame. Disclosure of Invention Technical Problem
• an object of the present invention to provide a method for generating a preamble sequence based on a PN sequence, and a method for performing time synchronization and estimating frequency offset in an OFDM communication system, in which a transmitter side of the OFDM communication system generates a preamble signal in frequency domain so as to make the PN sequence have good autocorrelation properties in time domain, and a receiver side of the OFDM communication system obtains a symbol synchronization and estimates a subcarrier frequency offset in time domain by using the PN sequence used to generate the preamble sequence.
• a method for generating a preamble sequence using a pseudo noise (PN) sequence at an OFDM transmission system including the steps of: generating a PN sequence having high autocorrelation value; and generating a preamble sequence by multiplying the PN sequence in frequency domain by an inverse matrix of an inverse fast Fourier transform (IFFT) matrix so as to make the preamble sequence become the PN sequence in time domain.
• PN pseudo noise
• a time synchronization method for an OFDM reception system including the steps of: storing PN sequence that is used to generate a preamble sequence at a transmitter side; and performing a moving sum with respect to OFDM reception signals by using the PN sequence, finding a position where the moving sum becomes maximum, and achieving the synchronization to the found preamble position.
• a method for obtaining a time synchronization and estimating a frequency offset using a PN sequence at an OFDM reception system including the steps of: finding a preamble position of an OFDM reception signal in time domain and obtaining a symbol synchronization to the found preamble position; and estimating a frequency offset of a subcarrier in time domain by multiplying the synchronized reception signal by a sequence given by differentially encoding the PN sequence.
• the OFDM communication system can estimate the time offset and wider range of the frequency offset effectively and simultaneously by designing the preamble sequence in frequency domain such that the preamble sequence becomes the PN sequence in time domain.
• PAPR can be remarkably reduced by designing the preamble sequence to be the PN sequence in time domain.
• the time synchronization and the frequency offset estimation can be achieved rapidly and easily.
• interference from other users can be reduced.
• the integer part and the fraction part of the frequency offset can be estimated at the receiver side, so that the estimation range of the frequency offset is wide.
• FIG. 1 illustrates an OFDM frame in time domain in an OFDM communication system
• FIG. 2 illustrates a structure of a preamble in an OFDM frame in accordance with an embodiment of the present invention
• FIG. 3 is a flowchart illustrating a method for generating a preamble sequence using PN sequence in an OFDM transmission system in accordance with an embodiment of the present invention.
• FIG. 4 is a flowchart illustrating a method for obtaining time synchronization and estimating frequency offset using preamble sequence in an OFDM reception system in accordance with an embodiment of the present invention. Best Mode for Carrying Out the Invention
• FIG. 1 illustrates a structure of an OFDM frame in time domain in an OFDM communication system.
• Fig. 2 illustrates a structure of a preamble in an OFDM frame in accordance with an embodiment of the present invention.
• Fig. 3 is a flowchart illustrating a method for generating a preamble sequence using PN sequence in an OFDM transmission system in accordance with an embodiment of the present invention.
• a preamble with a repetitive structure in time domain is transmitted and data is then transmitted.
• PN sequence is transmitted only on odd subcarriers in frequency domain, but not on even subcarriers.
• the preamble sequence has peak to average power ratio (PAPR) which causes non-linearity of an amplifier in a transmitter.
• PAPR peak to average power ratio
• the present invention designs a preamble sequence in time domain, instead of frequency domain.
• step 301 in order to generate a preamble sequence, a PN sequence having good autocorrelation property, i.e., high autocorrelation value, is generated.
• a PN sequence having good autocorrelation property i.e., high autocorrelation value
• other sequences having good autocorrelation properties may also be used as the preamble sequence.
• the PAPR that is the biggest problem in the OFDM becomes minimal, i.e., "1". Therefore, more margin can be provided with respect to the transmission power limitation due to PAPR and thus the preamble signal can be transmitted with larger power. Consequently, the synchronization can be achieved more exactly in high SNR.
• the OFDMA uplink system since an amplifier of a mobile station has inferior performance to a base station, the OFDMA uplink system is more sensitive to PAPR. Therefore, much more advantages can be obtained if the preamble sequence having a minimal PAPR is generated.
• a preamble sequence is generated by multiplying the PN sequence by an inverse matrix of an inverse fast Fourier transform (IFFT) matrix in frequency domain so that the PN sequence can become the preamble sequence in time domain.
• IFFT inverse fast Fourier transform
• NxN IFFT matrix M For OFDM symbol having N number of data symbols, NxN IFFT matrix M can be expressed as Eq. (1) below
• Fig. 2 illustrates a structure of the frequency domain preamble. That is, "p" in the frequency domain is multiplied by a Fourier transform matrix prior to transmission and converted into time domain signal, and it is transmitted as the PN sequence "s".
• the time synchronization is achieved in time domain. Therefore, it is easy to design the preamble sequence that is effective to the synchronization algorithm. Also, when the PN sequence is used in time domain, the time synchronization can be much effectively achieved due to the good autocorrelation property of the PN sequence. In the case of the uplink, several users try to access the base station at the same time in an initial operation, other user signals interfere with the user signal being synchronized, thus degrading the synchronization performance. However, this can reduce other user interferences by allocating different PN sequences to the users.
• FIG. 4 is a flowchart illustrating a method for obtaining time synchronization and estimating frequency offset using the preamble sequence in the OFDM reception system in accordance with an embodiment of the present invention.
• steps 400 to 408 when the OFDM reception system receives a signal, a time synchronization process of finding a preamble position in time domain is carried out.
• the receiver side knows that the preamble sequence is generated, in other words, designed, in a PN sequence in time domain and then transmitted, the receiver side performs a moving sum of the PN sequence previously stored in a memory and the reception signal, and finds a maximum value position.
• the PN sequence is used to generate the preamble at the transmitter side. Then, the receiver side adjusts the time synchronization to the maximum value position.
• an observation k window includes N sample values and thus an observed reception signal vector r ⁇ having ⁇ as a starting point of a moving sum is expressed as Eq. (3).
• V ⁇ Y f ⁇ f ⁇ + ⁇ ' ' ' f ⁇ +N-2 F ⁇ -N ⁇ ⁇ ] T
• T represents a transposed matrix
• the time synchronization can be obtained by finding ⁇ that maximizes X( ⁇ ).
• steps 400, 402 and 406 while increasing ⁇ from zero, the position where the moving sum becomes maximum is found while performing a moving sum with respect to the OFDM reception signal of Eq. (4) using the PN sequence. Then, the time synchronization is achieved using the found position as the starting position of the preamble.
• steps 404 and 408 in order to reduce the time necessary to find ⁇ , a threshold value TH ⁇ is set, and the time synchronization is achieved using ⁇ that is given when
• the influence of channel is small in multi-path environment. Also, the fraction part of the frequency offset can be estimated through the repetitive structure in time domain. On the contrary, in the method of the present invention, the influence of channel cannot be directly reduced because the channel information is not known in multi-path environment. However, using the good autocorrelation properties of the PN sequence, the interference caused by other channel delays can be remarkably reduced.
• the number of samples used for estimation is N/2 because of the repetitive structure.
• the number of samples is N. Because the mean estimation error is increased in inverse proportion to the number of the samples, the present invention has a merit of about 3 dB in view of the number of the samples.
• the present invention does not have the repetitive structure, the frequency offset has to be measured in time domain using other methods.
• the method for estimating the frequency offset in accordance with the present invention will be described. [59] First, a sequence of Eq. (5) below is defined.
• a differentially encoded C is also a sample of the PN sequence.
• the frequency offset ( ⁇ ) can be estimated as Eq. (7) below.
• the frequency offset of the subcarrier is estimated in time domain by multiplying the reception signal synchronized through the time synchronization procedures (steps 400 to 480) by the sequence given by differentially encoding the PN sequence.
• the method of the present invention has the estimation range of the frequency offset ( ⁇ ) of -N/2 ⁇ N/2. That is, compared with the related art, the estimation range of the frequency offset is widened. Also, while the number of the samples averaged in the related art is N/2, the preset invention can average the interference using (N-I) number of the samples. In addition, because C has the PN sequence, the interference removal effect can be obtained.
• step 412 a compensation operation is performed using the time offset ( ⁇ ) correct and frequency offset ( ⁇ ) obtained through the time synchronization and frequency correct offset estimation procedures. Since the compensation method is well known, its detailed description will be omitted.
• the above-described methods in accordance with the present invention can be stored in computer-readable recording media.
• the computer-readable recording media may include CD ROM, RAM, ROM, floppy disk, hard disk, optical magnetic disk, and so on. Since these procedures can be easily carried out by those skilled in the art, a detailed description thereof will be omitted.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Synchronisation In Digital Transmission Systems (AREA)

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• 2005
  • 2005-11-08 KR KR1020050106554A patent/KR100723634B1/ko not_active IP Right Cessation
• 2006
  • 2006-09-11 EP EP06798731A patent/EP1946469A1/de not_active Withdrawn
  • 2006-09-11 WO PCT/KR2006/003602 patent/WO2007055469A1/en active Application Filing

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